COBE

DIRBE Science Images

The images shown below were created from the
COBE DIRBE data products .
The colors generally do not map linearly into sky brightness.
Use the original data products for quantitative analysis.
Additional images are available in the COBE
Slide Set.

To view the original images, click on the postage-stamp
versions:

1.25, 2.2, and 3.5 micron Solar elongation angle = 90 deg
Maps - Galactic coordinate Mollweide projection maps of the
entire sky as seen by the DIRBE at a fixed angle relative
to the Sun. Stars concentrated in the Galactic plane (horizontal
feature) dominate the images at these wavelengths. Dust in
the Milky Way absorbs and scatters starlight, producing the
dark band that runs through the Galactic center in the 1.25
micron image; this "extinction" effect diminishes with increasing
wavelength.

False-color image of the near-infrared sky as seen by the
DIRBE. Data at 1.25, 2.2, and 3.5 µm wavelengths are
represented respectively as blue, green and red colors. The
image is presented in Galactic coordinates, with the plane
of the Milky Way Galaxy horizontal across the middle and the
Galactic center at the center. The dominant sources of light
at these wavelengths are stars within our Galaxy. The image
shows both the thin disk and central bulge populations of
stars in our spiral galaxy. Our Sun, much closer to us than
any other star, lies in the disk (which is why the disk appears
edge-on to us) at a distance of about 28,000 light years from
the center. The image is redder in directions where there
is more dust between the stars absorbing starlight from distant
stars. This absorption is so strong at visible wavelengths
that the central part of the Milky Way cannot be seen. DIRBE
data will facilitate studies of the content, energetics and
large scale structure of the Galaxy, as well as the nature
and distribution of dust within the Solar System. The data
also will be studied for evidence of a faint, uniform infrared
background, the residual radiation from the first stars and
galaxies formed following the Big Bang. For more information
about the Milky Way at wavelengths ranging across the entire
electromagnetic spectrum, see the Multiwavelength
Milky Way web site.

4.9, 12, 25, and 60 micron Solar elongation
angle = 90 deg Maps - Thermal emission from star-heated
dust in the Milky Way and interplanetary dust heated by the
Sun dominates the images at these wavelengths. The S-shaped
feature is the ecliptic plane, in which, like the planets,
the interplanetary dust is concentrated. The oval-shaped brightness
discontinuity is an artefact of the way the maps were prepared,
not a feature in the infrared sky. Specifically, the discontinuity
corresponds to a path difference through the interplanetary
dust cloud as adjacent positions in the sky were observed
from DIRBE's vantage point in Earth orbit with the Earth on
opposite sides of the Sun.

100, 140, and 240 micron Solar elongation
angle = 90 deg Maps - Thermal emission from relatively
cool interstellar dust warmed by stars in the Milky Way dominates
at these wavelengths. At high Galactic latitudes, interstellar
"cirrus" clouds are apparent. Emission from the solar system
dust ("zodiacal emission") is strongest at 25 microns but
remains in evidence in the 100 micron image, and to a lesser
degree at the longer wavelengths.

All of the Solar elongation angle = 90 deg Maps are shown with logarithmic
intensity scales. The following table gives the minimum and maximum log(I) for
each DIRBE photometric band.

Wavelength (microns)

min log (I, MJy/sr) (black)

max log (I, MJy/sr) (white)

1.25

-0.8

1.1

2.2

-0.95

1.3

3.5

-1.1

1.1

4.9

-0.5

0.75

12

1.0

1.7

25

1.2

1.91

60

0.7

1.91

100

0.42

2.0

140

0.05

2.71

240

0.05

2.71

The zodiacal and Galactic emission must be precisely modeled and subtracted in
order to detect the relatively faint Cosmic Infrared Background which the DIRBE
was designed to find. Secondary DIRBE objectives include studies of these astrophysical
foreground components.

Annual Average Maps at 3.5, 25, 100, and 240 microns - Galactic
coordinate Mollweide projection maps of the entire sky at
four wavelengths showing emission from stars and dust in the
Galactic plane (horizontal feature) and light scattered and
emitted by dust in the solar system (S-shape).

100 micron Weekly
Sky Maps for mission weeks 4 to 44, plus Annual Average Map
- shows sky coverage each week of the DIRBE mission over
the period during which the COBE cryogen supply lasted. As the
Earth, with COBE in orbit, revolved around the Sun, DIRBE viewed
the sky from an ever-changing vantage point in the solar system,
enabling light reflected and emitted by the interplanetary dust
cloud to be modeled.

DIRBE scan track superposed on 100 micron Annual Average Map
and 100 micron intensity from the corresponding segment of
Time-ordered Data - DIRBE scanned the sky in a helical
pattern that resulted from the spin and orbital motion of
the COBE satellite and the "look direction" of the telescope,
which was 30 degrees from the spin axis. The scan segment
depicted covers two COBE spin cycles, or about 150 seconds,
during a time when the DIRBE field of view swept through the
Sco-Oph region (bright area above the Galactic center in the
figure) and passed near the North Galactic Pole, where the
emission is faint. The brightness of the sky at 100 microns
measured during this interval is shown as a graph of intensity
vs. time, as given in the DIRBE
Time-ordered Data product. On the abscissa, the unit of
time is 1/8th of a second. A related gif
image shows intensity vs. time at four wavelengths
- 3.5, 25, 100, and 240 microns - during the same period.
Zodiacal emission is evident at 25 and 100 microns; the bumps
at about 470 and 1040 time units correspond to ecliptic plane
crossings. Stars give rise to the spikes seen at 3.5 microns.
The signal-to-noise ratio is clearly worse at 240 microns
than at the other wavelengths.

The following two figures were provided by Dr. Henry T. Freudenreich
and are described in his paper on The Shape and Color of
the Galactic Disk (
1996, ApJ, 468, 663). The Galactic plane runs horizontally
through each figure. The Galactic center lies along the 0
degree meridian and the anti-center direction appears near
the left side of each map. The 90 degree meridian is labeled
for scale.

J-K and 240 µm Maps Top: A sky map of the ratio of the surface brightness
in the DIRBE J band (1.25 µm) to the surface brightness
in the DIRBE K band (2.2 µm), after the zodiacal light
has been subtracted. The range is 0.8 to 1.4. Starlight dominates
the sky brightness at these wavelengths. The darker areas
in this image exhibit the light scattering and absorbing effects
of interstellar dust; those effects are greater at shorter
wavelengths.

Bottom: The surface brightness at 240 µm. At
this wavelength, in the far-infrared, stars are invisible
and we see thermal emission from dust heated by starlight.
The range of surface brightness is 0 to 115 MJy/sr. This map
is bright where the top map is dark because the same interstellar
dust clouds that absorb background starlight at near-infrared
wavelengths are warmed by this absorption to about 20 degrees
Kelvin, making them sources of far-infrared emission.

K-L and 240 µm Maps Top: A sky map of the ratio of the surface brightness
in the DIRBE K band (2.2 µm) to the surface brightness
in the DIRBE L band (3.5 µm), after the zodiacal light
has been subtracted. The range is 1.5 to 2.1. This map is
darker where there is more dust along the line of sight, like
the J-K map, but it also shows filamentary structure absent
from the J-K map, a sign of dust emission at 3.5 µm,
which requires a temperature of nearly 1000 Kelvins. An ultraviolet
photon could heat a large molecule (e.g., a polycyclic
aromatic hydrocarbon, or PAH), or a very small dust grain
to such a temperature.

Bottom: The surface brightness at 240 µm multiplied
by the sine of the Galactic latitude. This procedure accentuates
the relatively nearby interstellar clouds, such as the molecular
clouds in Orion (lower far left), Taurus (to the right of
Orion) and Ophiuchus (above Galactic center). When scaled
by the sine of latitude, the map intensity ranges from 0 to
13 MJy/sr. Comparison of this map to the K-L map shows that
regions containing cool dust tend to coincide with regions
containing hot, small dust grains. The molecules or tiny grains
seem to be a general component of the interstellar dust.